1. Field of the Invention
This invention concerns an optical disk apparatus suitable for detecting tracking error signals by the push-pull method, and the construction of an optical disk suitable for detecting tracking error signals by the push-pull method and focusing error signals by the astigmatism method.
2. Description of the Prior Art
FIG. 19 shows a conventional optical disk apparatus. A light beam from a laser light source 1 advances via a collimator lens 2, a mirror 3 and an objective lens 4 so as to form a spot on an optical disk OD. Light reflected from the optical disk OD passes through a beam splitter 5 and a condensing lens 6 to a photodetector 7. The objective lens 4 and the mirror 3 are installed in a head 8 which is free to slide in the radial direction of the optical disk, all devices from the laser light source 1 to the beam splitter 5 being fixed.
The photodetector 7, as shown in FIG. 20, comprises two regions A, B divided by a boundary line corresponding to the tangential direction of the optical disk, the difference between the output from these two regions A, B giving a tracking error signal according to the push-pull method.
The distribution of reflected light condensed on the photodetector 7 varies due to diffraction produced by the positional relationship of the pits or grooves and the spot on the optical disk.
FIG. 21 shows the light distribution on the photodetector when a light beam with no aberration is incident. In the figure, the x axis corresponds to the radial direction and the y axis to the tangential direction of the optical disk.
FIG. 21(a), (b) and (c), respectively, show the light distribution on the photodetector in: (a) the case wherein the spot on the optical disk is shifted in the +x direction from the center of a track; (b) the case wherein the spot is in the center of a track; and (c) the case wherein the spot on the optical disk is shifted in the -x direction from the center of a track. The variation of the light distribution due to a tracking error is asymmetrical only in the radial direction, and it is symmetrical in the tangential direction.
As the light distribution varies in this manner, it is possible to detect a shift between the spot and a track in the x direction, i.e. a tracking error, by measuring the difference of output from the two regions A, B of the photodetector 7.
The mirror 3 is free to rotate in the direction of the arrow in FIG. 19, and its angle is controlled by means of the tracking error such that the spot on the optical disk is in the center of a track.
In the aforesaid conventional optical disk apparatus, however, the balance in the light detected in each photodetecting region may be upset and the tracking error signal may contain an offset in addition to the ordinary error even if there is no change in the light distribution. An offset is produced when the entire spot on the photodetector is displaced due to the displacement of the optical path of the reflected light beam when the light beam is inclined relative to the optical disk as a result of a deformation of the disk or an inclination of the beam, and the displacement includes a component in a direction corresponding to the radial direction of time disk.
Further, when the mirror 3 is inclined without moving the head 8 so that the spot is moved in a radial direction, the optical path of the reflected light beam is shifted by an amount compared to the case when the mirror is in its reference position as shown in FIG. 22. The entire spot on the photodetector is then displaced from the reference position shown by the solid line to the position shown by the broken line in FIG. 23, and the tracking error signal will contain an offset.
FIG. 24 is a graph showing the relation between the position of the spot on a disk relative to a track, and the tracking error signal. The horizontal axis is the shift amount of the center of a spot from the center of a track, and the vertical axis is the tracking error signal TE. The solid line in the figure shows the tracking error signal when the light beam is reflected vertically from the optical disk, and the broken line shows the tracking error signal when the mirror 3 is inclined and the spot on the disk is shifted by 10 tracks from the aforesaid track.
If the optical disk and light beam are inclined relative to each other, the tracking error signal curve is shifted overall as the mirror inclination angle increases, and the actual shift amount and the signal do not correspond. Even if a servocontroller based on the tracking error signal is used, therefore, the position of the spot cannot be accurately controlled.